1. Field of the Invention
This invention relates to an optical switch used in fields of optical fiber communication, optical information processing, etc., and more particularly to a semiconductor waveguide type optical switch.
2. Description of the Related Art
An optical switch is one of optical circuit parts indispensable for switching terminals, optical fiber cables, etc., in an optical communication system and an optical information processing system. Among the optical switches, those of waveguide type are desirable from the viewpoints of easy integration and cascading. Hitherto, various waveguide type electro-optical switches such as the directional coupler, Mach-Zehnder, and total reflection types have been developed.
Two optical waveguides are disposed partially close to each other for the optical switch of the directional coupler type which uses an increase or decrease in the coupling degree between the two waveguides in the close portion or couple portion for switching the optical path from one of the waveguides to the other thereof. If the length of the couple portion equals to the "complete couple length" determined by the coupling strength, this optical switch causes the light emitting port to be switched in the couple portion thereof by inducing the difference between propagation constants on the two waveguides. The propagation constant change is caused by a change in the refractive index of one of the waveguides when an electric field is applied.
The optical switch of the Mach-Zehnder type has a structure where one waveguide is once branched into two portions which then are again coupled together. An electric field is applied to each branch of the waveguide to change the refractive index thereof, thereby generating a phase difference for the light passing through the branches. These branches of light are turned on or off in response to their phase differences when they are coupled together. The phase difference is caused by the refractive index change of each branch of the waveguide when the electric field is applied.
The optical switch of the total reflection type is made up of two waveguides which cross each other. An electric field is applied to the intersection of the waveguides to change the reflectance ratio, whereby total reflection within the waveguides is used to move a light on one of the waveguides to the other thereof. The reflectance ratio change is caused by the refractive index change of the waveguides when the electric field is applied.
With all the waveguide type electro-optical switches described above, an electric field is applied to the waveguide or waveguides to change the refractive index, however, recently an optical method has been watched as a method which has possibilities of switching operation at higher speed as compared with the waveguide type electro-optical switches. This optical method is to form a waveguide with a semiconductor and enter controlling light in the waveguide to cause the semiconductor to absorb the controlling light, thereby causing a refractive index change in any desired part of the waveguide.
With the above-described optical method, carriers are generated within the waveguide at the same time as the refractive index changes when the light is absorbed. To enable high-speed switching the carriers must be eliminated from within the waveguide for as short time as possible. Known as one of the methods for eliminating carriers to enable high-speed switching is a method of applying an electric field to the waveguide part where the refractive index is to be changed. (Refer to Japanese Non-Examined Patent Publication No. 4-3125 (Japanese Patent Application No. 2-104943)). Since the carriers are drawn out rapidly from the waveguide by the applied electric field, the recovery time from the refractive index change of the waveguide is shortened to enable high-speed switching.
To reduce switching energy, it is necessary for the controlling light to cause a sufficiently large change in the refractive index on to be made on the waveguide and also necessary for controlled light to be subjected to large phase modulation due to the refractive index change. For this purpose, it is preferably that the controlling light together with the controlled light is entered in the waveguide so as to change the phase of the controlled light. Further, it is preferable that the length of the waveguide part where the refractive index is to change is set long.
As described above, a semiconductor waveguide type optical switch requires that the controlling light entered together with the controlled light in the waveguide is absorbed in a predetermined part of the waveguide and pass through other parts thereof without absorption. In other words, the wavelength of the optical absorption edge in the predetermined part of the waveguide where the controlling light should be absorbed must be made shorter than the wavelength of the optical absorption edge in any other part thereof. To meet this requirement, it is possible to form the waveguide portion where the refractive index is to be changed and other portions thereof with semiconductors which differ in energy gap; however, this cannot easily be implemented.